Improvements have been proposed in the tan .delta. of tire tread rubber for reducing the fuel consumption of a vehicle. Specifically, separate mixing of ingredients and use of terminal-modified rubber have been proposed.
For example, Japanese Examined Patent Publication (Kokoku) No. 5-1298 discloses a rubber composition further improving the impact resilience and tensile strength of a vulcanizate including a conjugated diene polymer having an aromatic tertiary amino group in the terminal portion of the conjugated diene polymer. On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 55-10434 discloses, during the production of a rubber composition for a tire tread comprising blending a starting rubber composed of an amorphous 1,2-polybutadiene and natural rubber and/or polyisoprene rubber (or additionally including a partially conjugated diene rubber) with carbon black, first mixing a starting rubber containing at least 25% by weight of the 1,2-polybutadiene and the carbon black in a specific ratio, then adding and mixing the remaining starting rubber so as to improve the fuel economy and safety of a car.
Further, Japanese Examined Patent Publication (Kokoku) No. 2-129241 proposes the improvement of the tensile strength and abrasion resistance of the vulcanizate by mixing a terminal-modified conjugated diene polymer with carbon black in an organic solvent.